Syrian rebels and the al Qaeda-affiliated Nusra Front have mounted an offensive against Syrian government forces and on Saturday took a strategic hill south of Aleppo from government control, a monitoring group said.

The Britain-based Syrian Observatory for Human Rights said the offensive began on Friday, and Syrian government and allied forces were fighting to take back control of the territory and repel further attacks.

A fragile "cessation of hostilities" truce has held in Syria for over a month as the various parties try to negotiate an end to Syria's five-year-old civil war.

But the truce excludes Islamic State and Nusra Front, and air and land attacks by Syrian and allied forces continue in parts of Syria where the government says the groups are present.

Friday's attack began with three suicide bombings carried out by Nusra Front south of Talat al-'Iss hill. Rebel and Nusra Front forces then took control of the hill, the Observatory and a Nusra Front statement said.

Nusra Front said in the statement it ambushed government forces as they withdrew toward the town of Hadhar to the east.

The Observatory said the fighting has so far killed dozens of regime forces and at least 16 rebels and Nusra Front members.

Live footage broadcast on pro-opposition Orient News TV on Saturday showed the hill provides a vantage point across a wide area. It also lies near a main highway linking Aleppo to Damascus, Syria's capital, to the south.

"This is the dividing line, the front position of the Syrian army in the southern (Aleppo) countryside," a Syrian military source told Reuters.

"In south Aleppo, armed groups in coordination with Nusra Front attacked some military positions ... in the direction of Talat al-'Iss and the surrounding areas. Of course this is a clear breach of the truce," the source said.